This disclosure relates to high-pressure direct-injection (HPDI) fuel systems designed to inject a primary fuel, such as natural gas, and a pilot fuel, such as diesel, into a combustion chamber, for example a cylinder of a reciprocating internal combustion (IC) engine. More specifically, this disclosure describes an HPDI system that features a “limp mode” when the supply of natural gas is depleted or otherwise interrupted. Still more specifically, this disclosure describes a control system and method for generating and isolating high-pressure pilot fuel in an HPDI system when operating in a limp mode.
Heavy-duty IC engines that run on natural gas instead of diesel are desirable because natural gas has cost advantages and produces fewer emission products compared to diesel. Engines that burn natural gas may be spark-ignited or compression-ignited. Spark-ignited engines are available, but spark-ignited engines that run on natural gas have reduced efficiencies and lower torques at low speeds when compared with traditional diesel engines. However, spark-ignited engines that run on natural gas are commonly used for transit buses, delivery vehicles, shuttles, street sweepers and other applications that do not require high torque and low speeds.
Compression-ignited engines that burn natural gas also burn some diesel as a pilot fuel and are therefore referred to as dual-fuel engines. Fuel systems for dual-fuel compression-ignited engines come in two types: substitution systems and high-pressure direct injection (HPDI) systems. Substitution systems simply add natural gas to the intake air stream and decrease the amount of diesel fuel, thereby “substituting” a percentage of the diesel with natural gas. Drawbacks of substitution systems include reduced power output because introducing natural gas into the intake air system reduces the amount of air drawn into the engine, and the amount of natural gas that can be substituted is limited by engine knock limits to avoid premature detonation of a premixed charge of natural gas and air. Engines equipped with substitution systems will run either on natural gas and substantial amounts of diesel or pure diesel (with no natural gas), but will not run primarily on natural gas. The substitution percentage of diesel with natural gas typically ranges between 0 and 65%, dependent upon the load and operating conditions. While substitution systems account for the majority of diesel/natural gas engines in use today, the inability to run on natural gas without substantial amounts of diesel renders substitution systems less environmentally friendly than HPDI systems. Further, substitution systems do not provide the fuel cost savings provided by HPDI systems when natural gas is less expensive than diesel fuel.
HPDI systems burn primarily natural gas with a small amount of diesel as a pilot fuel. The diesel is injected into the cylinder just prior to the injection of high-pressure natural gas to provide the ignition. Typically, the diesel amounts to less than 10% of the combusted fuel and therefore the emission reduction is substantial. Engines equipped with HPDI fuel systems offer power, torque and efficiency similar to that of traditional diesel engines. Further, a traditional diesel engine may be converted to an HPDI engine by replacing the diesel fuel system with an HPDI fuel system.
Current HPDI systems may run on diesel only, for those situations where the natural gas supply is depleted or natural gas is otherwise not available, or for extremely cold starts when the engine is too cold to effectively vaporize the natural gas stored in liquefied form, as disclosed in CA 2849623. During normal operations, when diesel is used as a pilot fuel, the diesel and natural gas are pressurized to a normal system pressure of about 30 MPa. However, when the engine is operating in a limp mode (also known as a limp-home mode, run-on diesel (ROD) mode and diesel only mode (DOM)), injecting diesel at the normal operating pressure provides only about 10% of the engine power. Operating at such a reduced power is very disadvantageous for some applications, such as mine haul trucks, where substantial power may be needed to move a truck off the haul road. Further, trucks with HPDI systems may need to travel substantial distances or climb steep inclines to reach a site where the natural gas supply can be replenished.
A solution to this problem would be to increase the pressure of the diesel from the normal operating pressure of about 30 MPa to a higher pressure of about 100 MPa while operating in the limp mode, but certain components of an HPDI system, such as the pressure regulator, cannot withstand the excessive force imbalance between the normal operating pressure and the high pressure needed to run on diesel only. Thus, a need exists for an HPDI fuel system and method that delivers high-pressure diesel (or high-pressure secondary fuel) to the engine when the engine is operating in a limp mode without compromising components needed to operate the engine in a normal operating mode.